During DPReview's tour of the Utsunomiya L lens factory, they had the opportunity to interview the 3 men that oversee the facility. This factory produced every L lens, cinema lens and broadcast lens in the Canon lineup.
From the interview:
What percentage of L lenses are manufactured in the Utsunomiya lens plant?
Because this is the ‘mother' factory, 100% of L lenses are made here.
How many different lenses can be manufactured simultaneously in this plant?
Basically, we create all lenses every day [including L-series EF, Cinema EOS and broadcast]. The only exception is some of the broadcast lenses.
Which lenses in particular are the most difficult to manufacture and why?
Any large super telephoto lenses because of the size of the glass elements. In terms of skill required for lens assembly: the TV broadcast lenses are most difficult.
How many lenses are produced at this lens plant every year, both in terms of types of lenses and total units?
We do not disclose total production for this plant. That said, Canon has produced a total of 120 million lenses over the years. Of course, many of those are kit lenses, which are not produced here, but in our facility in Taiwan. Read the full interview
It's nice to see Canon opening up a bit about how they manufacture what we think are consistently the best lenses in the world (No bias, honest!).